Feedlot performance, feed efficiency reranking, carcass traits, body composition, energy requirements, meat quality and calpain system activity in Nellore steers with low and high residual feed intake

The aim was to evaluate growth, carcass traits, feed efficiency reranking, body composition, calpain system activity and meat quality in Nellore steers that were phenotypically ranked for high and low residual feed intake (RFI). Seventy-two Nellore steers (16-21 month-old, 334 +/- 19 kg BW) had free access to a feedlot diet for 70 d (feeding period 1, P1). Daily dry matter intake (DMI), body weight gain (ADG) and ultrasound carcass traits were measured individually. The 12 lowest and the 12 highest RFI steers were classed as low- and high-RFI groups and were fed for a second feeding period (feeding period 2, P2). Spearman's rank correlation was performed for RFI and gain-to-feed ratio (G:F) measured over P1 and P2. The carcass traits, meat quality and calpain system activity were evaluated at slaughter, and body composition was determined. In P1, low-RFI steers had greater G:F (0.159 vs. 0.134; P < 0.001). lower DMI (9.30 vs. 11.1 kg/d; P < 0.0001), lower RFI (-0.80 vs. 0.85 kg/d; P < 0.0001), and tended to have lower rates of rump fat gain (4.48 vs. 6.05 mm; P=0.06), but no differences were observed for ADG, BW, and other traits measured by ultrasound (P > 0.05). In P2, no differences between RFI classes were observed for G:F and DMI as a percentage of BW (P > 0.05), and small differences were observed for DMI (8.25 vs. 8.99 kg/d, P < 0.05) and RFI (-0.28 vs. +0.29 kg/d, P=0.06). The ranking correlations for RFI and G:F measured consecutively in P1 and P2 were low to moderate (r=0.11-0.40). Low-RFI steers had lower requirements of metabolizable energy (ME) for maintenance (131 vs. 160 Mcal/kg EBW0.75 d(-1), P < 0.05), but no differences were observed for fat and protein gain, retained energy and efficiency of ME use for gain (P > 0.05). No differences between low- and high-RFI steers were observed for carcass traits at slaughter; however, low-RFI steers had 8.1 kg less gastrointestinal fat than high-RFI steers. There were no RFI effects on meat shear force and the activities of mu-calpain, m-calpain and calpastatin (P > 0.05). High-RFI steers had greater MFI in 1 d aged LM (53.9 vs. 40.8, P < 0.05). The variation in feed efficiency between high- and low-RFI Nellore steers is related to differences in energy requirements and deposition of fat on internal organs. The ranking for feed efficiency is altered as cattle become older. The selection for improved RFI in Nellore cattle may reduce feed requirements for beef production without affecting meat tenderness and enzymatic activity of the calpain system. (C) 2012 Elsevier B.V. All rights reserved.